Creating a high definition digital image requires a large amount of data. As stated by John Wiseman in An Introduction to MPEG Video Compression;                  One of the formats defined for HDTV broadcasting within the United States is 1920 pixels horizontally by 1080 lines vertically, at 30 frames per second. If these numbers are all multiplied together, along with 8 bits for each of the three primary colors, the total data rate required would be approximately 1.5 Gb/sec. Because of the 6 MHz channel bandwidth allocated, each channel will only support a data rate of 19.2 Mb/sec, which is further reduced to 18 Mb/sec by the fact that the channel must also support audio, transport, and ancillary data information. As can be seen, this restriction in data rate means that the original signal must be compressed by a figure of approximately 83:1.        
Although this is only a single example of a specific format, it serves to illustrate that compressing digital images is an area of great interest to those who provide digital transmissions.
Throughout the specification and claims, we will be using the term MPEG (Motion Picture Expert Group). MPEG is a generic reference to a family of international standards, which define how to encode visual and audio information in a digital compressed format. MPEG is utilized in a wide variety of applications, including DVD (Digital Video Discs) and DVB (Digital Video Broadcasting).
The MPEG standards specify exactly the format in which the compressed data is to be transmitted. A key feature of MPEG is that it can compress a video signal into a fraction of its original size. MPEG achieves a high compression for video by storing only the changes from one video frame to another, instead of each entire frame. This compression process is known as encoding and is done by an encoder. At the receiving end of an MPEG transmission, there exists a decoder, which decodes the transmission and restores it as best it can to the video signal originally encoded.
There are two major MPEG standards: MPEG-1 and MPEG-2.The most common implementations of the MPEG-1 standard provide video quality slightly below the quality of conventional VCR (Video Cassette Recorder) videos. MPEG-2 provides higher resolution, with full CD quality audio. This is sufficient for the major TV standards, including NTSC (National Standards Television Committee) and HDTV (High Definition Television).
Of the series of MPEG standards that describe and define the syntax for video broadcasting, the standard of relevance to the present invention is ISO/IEC IS 13818-2, ITU-T Recommendation H.262, titled “Generic coding of moving frames and associated audio information: Video,” which is incorporated herein by reference and is hereinafter referred to as “MPEG-2”.
An MPEG video transmission is essentially a series of pictures taken at closely spaced time intervals. Often a picture may be quite similar to the one that precedes it or the one that follows it. For example, video of waves washing up on a beach would change little from picture to picture. Except for the motion of the waves, the beach and sky would be largely the same. Once the scene changes, however, some or all similarity may be lost. The concept of compressing the data in each picture relies upon the fact that many images do not change significantly from picture to picture. Thus, considerable savings in data transmission can be made by transmitting only the differences between pictures, as opposed to the entire picture. In the MPEG-2 standard a picture is referred to as a “frame”. This is terminology we will use from now on.
If an MPEG-2 stream is to be viewed immediately as it is received, the communication channel must have enough bit rate capacity to provide the series of frames at a real-time rate. Bit rate is the number of digital bits which a communication channel can transmit per second. Alternatively, frames can be encoded to a size suitable for a channel of a given bit rate. MPEG-2 encoding allows the size of each frame to be adjusted by varying quality, thus smaller frames may be achieved at the expense of lower quality. The objective of an MPEG-2 encoding scheme is to maximize quality for the available bit rate.
Consider a system in which high-quality MPEG-2 video is to be played directly from a storage medium; DVD is an example. The video quality is high, as instantaneous bit rate is of relatively low concern. Consider now that the stored high-quality video is to be communicated across a channel of constrained bit rate such as a telephone line. Some device must re-encode each frame (with potential reduction of quality) so the sequence of frames may be transmitted in real time within the available bit rate. Such a device is known as a transcoder. A transcoder converts an encoded bit stream of one bit rate to a lower bit rate, and in doing so, changes the content of the encoded bitstream.
MPEG-2 video produces a variable bit rate stream. Therefore, a buffer between the transcoder and the decoder is necessary to achieve a constant bit rate transfer. The size of the buffer will determine the frame size variation allowable. Consequently, an MPEG-2 transcoder must monitor buffer fullness and control the bit rate of each frame to avoid buffer underflow and/or overflow.
Methods for achieving rate reduction already exist. One such example is the method disclosed in U.S. Pat. No. 6,208,688, which makes use of a method known as requantization. The, 6,208,688 patent does not consider video transcoder buffers, which are necessary to achieve constant bit rate operation of an MPEG-2 video transcoder. Further, because it addresses only bit rate it fails to consider image quality. By doing so, the choice of a particular requantization step size to achieve a target bit rate may produce a bit stream with worse image quality than that of a lower bit rate stream.
Precisely achieving a target bit rate while maintaining good video quality in a transcoder presents many challenges. There is a need for a simple rate control method in a transcoder which monitors the video buffers to avoid underflow/overflow, and achives a target bit rate at the best possible image quality. The present invention addresses this need.